The present invention relates to a packaging machine and a method for filling bags. The packaging machine according to the invention may be provided for bagging all kinds of bulk goods. This kind of system is particularly preferred for bagging fine-grain products, i.e. for bagging fine and dusty products requiring lengthy filling and in particular compacting times.
A great variety of packaging machines for filling open-mouth bags have been disclosed in the prior art. For example so-called FFS packaging machines (“Form Fill Seal packaging machines”) tend to be used for efficiently filling bulk materials into open-mouth bags. In these FFS packaging systems the open-mouth bag is manufactured on the machine or in an immediately preceding device. The machine has a roll of tubular sheet assigned to it from which open-mouth bags are continuously manufactured during operation as required. A considerable advantage of these FFS packaging machines is that the open-mouth bags can be manufactured in the actually required lengths. There is no need to use prefabricated bags which are also more expensive.
FFS packaging machines process open-mouth bags of plastic sheets which may be waterproof. This is why open-mouth bags filled with hygroscopic materials—such as cement—are suitable for open-air storage after closing since their contents are reliably protected from moisture.
The drawback of known packaging machines for filling open-mouth bags is their limited filling capacity in particular when bagging dusty, fine-grain products since these products must as a rule be compacted to provide a firm package containing the smallest possible amount of air. Trapped air moreover reduces the stacking capability.
Rotary packaging machines in indexed operation can basically ensure increased filling rates since e.g. in one position up to e.g. 90% of the intended fill quantity is filled in high speed while in the next angular position the remaining quantity is filled in slow speed. In the subsequent angular position the product in the bag can be compacted further and deaerated. The following step provides for discharge and then for the appending of another open-mouth bag.
In the case of a packaging machine provided for gross weighing the bag appended to the filling spout must be continuously weighed during the filling process. Filling is stopped as the target weight has been reached. The drawback of this is that vibrating devices for compacting the filled product do not only act on the bag but the vibrations are also transmitted directly to the scales so that incorrect weights may be measured. Therefore the vibrating device is switched off during filling. This alternating weighing and vibrating, however, reduces the filling rate.
Another option is filling by way of a differential weighing device. Such a system has been disclosed e.g. in DF 10 2008 043 545 A1. Therein the filling rate is increased by employing a bagging carousel having two differential weighing devices for high seed filling and a differential weighing device for low speed filling. However, despite the three differential weighing devices, the filling rate is still rather low. This system shows another drawback in that in the worst-case scenario, any inaccuracies of all three of the differential weighing devices used may add up so that it is difficult to ensure precise weighing. Moreover in the case of e.g. poorly flowing bulk goods, caking may occur within the product path. When part of the intended fill cakes at the wall then the currently processed container will be underweight. When caking detaches at a later time then a following container will show some overweight.